Virtual colonic irrigation gives clear view of cancers

A kind of virtual colonic irrigation could help to reduce the occurrence of colorectal cancer.

Colorectal cancer must be caught early for the best chance of successful treatment, and the disease remains the second greatest cause of cancer deaths in the US.

But patients can be reluctant to take part in screening programmes which involve uncomfortable procedures. The traditional option is a colonoscopy, where a fibre-optic camera is passed through the anus to look for polyps.

A more recent alternative is virtual colonoscopy which use a CT scanner to construct a 3D model of the colon. But that still involves physically cleaning out, or "irrigating", a person's colon to get the best picture. See an example video of a virtual colonoscopy.

Virtual cleaning

But now even irrigating a person's colon can be done in a virtual environment. Jerome Liang and his team at the State University of New York at Stony Brook, US, have developed a way to automatically remove the faecal matter inside the colon from the CT scans. That leaves a clear image to reveal any polyps lurking within.

Patients must first eat a meal tagged with a dense chemical, such as barium sulphate. That creates a strong contrast between the faecal matter and the body tissue on a scan, making it easier to automatically filter out the colon's contents, says Liang.

"The ultimate goal is to virtually cleanse faecal materials with very little bowel preparation," he says.

Stool tagging

Ronald Summers' research group at the National Institutes of Health Clinical Center in Bethesda, Maryland, US, is also researching virtual colonoscopy techniques.

"I think [irrigation-free] CT colonography with or without computer-aided polyp detection is possible in the future," he says. "But it will be highly dependent on the quality of the stool tagging."

Although Liang agrees, he thinks that better software can also have a significant impact. Currently virtual colonoscopy can only detect relatively large polyps of over 5 millimetres in diameter.

Liang's new software improves those detection limits and can identify polyps under 5 mm by being able to differentiate between the body's tissues.

Mixed pixels

Existing colon-scanning software assumes that each pixel of the image contains only one type of tissue. But in reality it may contain two or more - for example, fat and muscle.

However, fat, muscle, faeces and bone each have a characteristic density on a CT scan, so if a pixel has a density that does not match any of those materials it likely contains a mixture.

Liang's software is able to identify the component parts of these mixed pixels, meaning it can more accurately draw the boundary between the colon and its contents to better reveal small polyps. When the team tested its software on CT scans of 52 cancer patients, they found 24 polyps below 5 mm in size.

That makes it ideal for mass screening of patients, says Liang, because it is approaching the accuracy of invasive methods, but involves almost no discomfort.

Cancer - Learn more about one of the world's biggest killers in our comprehensive special report.

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